Sensory responses of caudal trigeminal neurons to thermal and mechanical stimuli and their behavioural correlates in the rat

Electroacupuncture and moxibustion-like stimulation activate the cutaneous and systemic hypothalamic-pituitary-adrenal axes in the rat

OBJECTIVE

To determine whether electroacupuncture (EA) or moxibustion-like stimulation (MLS) can affect the cutaneous and/or systemic hypothalamic-pituitary-adrenal (HPA) axes.

METHODS

Rats were divided into Control, EA, 37°C MLS and 43.5°C MLS groups. EA and MLS were performed at bilateral ST36 or LI4. The expression of corticotropin-releasing factor (CRF), adrenocorticotropic hormone (ACTH) and the glucocorticoid receptor (GR) was detected in local cutaneous tissues at the site of ST36 and LI4 by immunohistochemical staining. In addition, levels of CRF, ACTH and corticosterone (CORT) in cutaneous tissue and plasma were determined.

RESULTS

Cutaneous expression of CRF, ACTH and GR significantly increased after EA at ST36, while only GR increased after 43.5°C MLS at ST36. The results of EA and MLS at LI4 were in parallel with those at ST36. In plasma, compared with the control group, the level of CORT increased after EA at ST36, while both ACTH and CORT were markedly increased after 43.5°C MLS. For LI4, plasma CRF and CORT increased after EA, while the levels of all three hormones increased following 43.5°C MLS. Notably, compared with the effect of EA, 43.5°C MLS at ST36 produced a more substantial increase in plasma CORT, and 43.5°C MLS at LI4 induced a more dramatic increase in plasma CRF and CORT.

CONCLUSION

Both EA and 43.5°C MLS can activate the cutaneous and systemic HPA axes of the rat. EA tended to activate the local cutaneous HPA, while 43.5°C MLS was more likely to activate the systemic HPA axis.

The effects of blocking N/OFQ receptors on orofacial pain following experimental tooth movement in rats

Objective

The aim of this study was to determine the effects of nociceptin/orphanin FQ peptide receptor (N/OFQ receptor) antagonist on orofacial pain induced by experimental tooth movement in rats.

Methods

A total of 36 male Sprague-Dawley rats weighing 200–300 g were divided into six groups: a control group, force group, force+saline intraperitoneal group, force+saline periodontal group, force+UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ-NH2; antagonist for N/OFQ receptor) intraperitoneal group, and force+UFP-101 periodontal group. Closed coil springs were ligated between the upper incisors and first molar to exert an orthodontic force (40 g) between the teeth. Injectable administration dosages were 30 μl saline or 30 μl saline containing 0.03 mg/kg UFP-101. Following the injections, orofacial pain levels were assessed through directed face grooming (mouth wiping). Statistical analyses were performed in SPSS 17.0 (Statistical Package for the Social Sciences) and p values less than 0.05 were considered as statistically significant.

Results

Orofacial pain levels were significantly higher in the force group than in the control group. Orofacial pain levels differed significantly between the force group, force+saline periodontal group and force+UFP-101 periodontal group, but were similar between the control group, force+UFP-101 intraperitoneal group and force+saline intraperitoneal group. Moreover, orofacial pain levels did not differ between the force group, force+saline intraperitoneal group and force+UFP-101 intraperitoneal group.

Conclusions

Periodontal, but not intraperitoneal, administration of UFP-101 could alleviate orofacial pain induced by experimental tooth movement in rats, suggesting that periodontal N/OFQ receptors participate in orofacial pain induced by experimental tooth movement.

Cell-type-specific recruitment of amygdala interneurons to hippocampal theta rhythm and noxious stimuli in vivo

Neuronal synchrony in the basolateral amygdala (BLA) is critical for emotional behavior. Coordinated theta-frequency oscillations between the BLA and the hippocampus and precisely timed integration of salient sensory stimuli in the BLA are involved in fear conditioning. We characterized GABAergic interneuron types of the BLA and determined their contribution to shaping these network activities. Using in vivo recordings in rats combined with the anatomical identification of neurons, we found that the firing of BLA interneurons associated with network activities was cell type specific. The firing of calbindin-positive interneurons targeting dendrites was precisely theta-modulated, but other cell types were heterogeneously modulated, including parvalbumin-positive basket cells. Salient sensory stimuli selectively triggered axo-axonic cells firing and inhibited firing of a disctinct projecting interneuron type. Thus, GABA is released onto BLA principal neurons in a time-, domain-, and sensory-specific manner. These specific synaptic actions likely cooperate to promote amygdalo-hippocampal synchrony involved in emotional memory formation.

Effects of drugs of abuse on putative rostromedial tegmental neurons, inhibitory afferents to midbrain dopamine cells

Recent findings have underlined the rostromedial tegmental nucleus (RMTg), a structure located caudally to the ventral tegmental area, as an important site involved in the mechanisms of aversion. RMTg contains γ-aminobutyric acid neurons responding to noxious stimuli, densely innervated by the lateral habenula and providing a major inhibitory projection to reward-encoding midbrain dopamine (DA) neurons. One of the key features of drug addiction is the perseverance of drug seeking in spite of negative and unpleasant consequences, likely mediated by response suppression within neural pathways mediating aversion. To investigate whether the RMTg has a function in the mechanisms of addicting drugs, we studied acute effects of morphine, cocaine, the cannabinoid agonist WIN55212-2 (WIN), and nicotine on putative RMTg neurons. We utilized single unit extracellular recordings in anesthetized rats and whole-cell patch-clamp recordings in brain slices to identify and characterize putative RMTg neurons and their responses to drugs of abuse. Morphine and WIN inhibited both firing rate in vivo and excitatory postsynaptic currents (EPSCs) evoked by stimulation of rostral afferents in vitro, whereas cocaine inhibited discharge activity without affecting EPSC amplitude. Conversely, nicotine robustly excited putative RMTg neurons and enhanced EPSCs, an effect mediated by α7-containing nicotinic acetylcholine receptors. Our results suggest that activity of RMTg neurons is profoundly influenced by drugs of abuse and, as important inhibitory afferents to midbrain DA neurons, they might take place in the complex interplay between the neural circuits mediating aversion and reward.

Activity of neurochemically heterogeneous dopaminergic neurons in the substantia nigra during spontaneous and driven changes in brain state

Dopaminergic neurons of the substantia nigra (SN) and ventral tegmental area (VTA) are collectively implicated in motor- and reward-related behaviors. However, dopaminergic SN and VTA neurons differ on several functional levels, and dopaminergic SN neurons themselves vary in their intrinsic electrical properties, neurochemical characteristics and connections. This heterogeneity is not only important for normal function; calbindin (CB) expression by some dopaminergic SN neurons has been linked with their increased survival in Parkinson's disease. To test whether the activity of CB-negative and CB-positive dopaminergic SN neurons differs during distinct spontaneous and driven brain states, we recorded single units in anesthetized rats before, during and after aversive somatosensory stimuli. Recorded neurons were juxtacellularly labeled, confirmed to be dopaminergic, and tested for CB immunoreactivity. During cortical slow-wave activity, the firing of most dopaminergic neurons was slow and regular/irregular and unrelated to cortical slow oscillations. During spontaneous cortical activation, dopaminergic SN neurons fired in a more regular manner, with fewer bursts, but did not change their firing rate. Regardless of brain state, CB-negative dopaminergic neurons fired significantly faster than CB-positive dopaminergic neurons. This difference in firing rate was not mirrored by different firing patterns. Most CB-negative and CB-positive dopaminergic neurons did not respond to the aversive stimuli; of those that did respond, most were inhibited. We conclude that CB-negative and CB-positive dopaminergic neurons exhibit different activities in vivo. Furthermore, the firing of dopaminergic SN neurons is brain state-dependent, and, unlike dopaminergic VTA neurons, they are not commonly recruited or inhibited by aversive stimuli.

Behavioural responses and expression of P2X3 receptor in trigeminal ganglion after experimental tooth movement in rats

OBJECTIVE

To explore the role of P2X(3) receptor in pain induced by experimental tooth movement.

DESIGN

Male Sprague-Dawley rats weighing 200-300g were used. P2X(3) receptor distribution in the caudal one-third portion of the trigeminal ganglion (TG) was studied by IHC. Next, the changes of P2X(3) expression were detected by Western blotting 4h, 1d, 2d, 3d, 5d, 7d, 14d after tooth movement. We then developed a behaviour pain model associated with directed mouth wiping. Finally, the effect of TNP-ATP on nociceptive-like behaviour was evaluated.

RESULTS

Our results showed that P2X(3) receptors were expressed mainly in small- and medium-sized cells and experimental tooth movement led to an increase in staining of mandibular P2X(3) receptors. In addition, following experimental tooth movement, the expression of P2X(3) receptor in TG was statistically significantly up-regulated from days 1 to 5, with a peak on day 3. It was also found that the time spent on directed mouth wiping was dramatically increased by experimental tooth movement from days 1 to 7. The rhythm change of P2X(3) receptor expression in TG and the mouth wiping behaviour were in concert with the initial orthodontic pain responses. The directed mouth wiping behaviour was modulated in a force-dependent manner and could be attenuated by peripheral and systemic morphine. Furthermore, peripherally administered TNP-ATP could exert an analgesic effect on this pain model.

CONCLUSION

These results suggest that directed mouth wiping behaviour can be a reliable measurement of pain following experimental tooth movement in rats. The P2X(3) receptor is important in the development and maintenance of tooth movement pain and thus may be peripheral targets for analgesics in orthodontic pain control.

Nitric oxide/cGMP pathway mediates orofacial antinociception induced by electroacupuncture at the St36 acupoint

The aim of the present study was to test the hypothesis that electroacupuncture (EA) at acupoint St36 induces antinociception by activation of the L-arg/NO/cGMP pathway. Nociception was produced by thermal stimuli applied to the face of Wistar rats and latency was measured by face withdrawal. Electric stimulation of acupoint St36 for 20 min induced antinociception in this model, which was maintained for 150 min. For comparison, a so-called dry needle group (DN) was used, which received needling at the same point without stimulation. The antinociception obtained by stimulation of acupoint St36 was only achieved when high frequency (100 Hz) was used, whereas low frequencies (5 and 30 Hz) were not capable of achieving this effect. Subcutaneous administration of both inhibitors of NO synthase (N-nitro-L-arginine) and guanylyl cyclase (ODQ) and intraperitoneal administration of specific antagonists of neuronal NO synthase (L-NNA) and inductible NO synthase (aminoguanidine) antagonized the antinociception induced by St36 stimulation. The results of this paper suggest that stimulation of acupoint St36 at high frequency induces antinociception, which seems to be related to L-arg/NO/cGMP pathway activation.

Antinociceptive effect and interaction of uncompetitive and competitive NMDA receptor antagonists upon capsaicin and paw pressure testing in normal and monoarthritic rats

We assessed whether intrathecal administration of the uncompetitive and competitive NMDA receptor antagonists ketamine and (+/-)CPP, respectively, could produce differential modulation on chemical and mechanical nociception in normal and monoarthritic rats. In addition, the antinociceptive interaction of ketamine and (+/-)CPP on monoarthritic pain was also studied using isobolographic analysis. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the tibio-tarsal joint. Four weeks later, the antinociceptive effect of intrathecal administration of the drugs alone or combined was evaluated by using the intraplantar capsaicin and the paw pressure tests. Ketamine (0.1, 1, 10, 30, 100, 300 and 1000 microg i.t.) and (+/-)CPP (0.125, 2.5, 7.5, 12.5, 25 and 50 microg i.t.) produced significantly greater dose-dependent antinociception in the capsaicin than in the paw pressure test. Irrespective of the nociceptive test employed, both antagonists showed greater antinociceptive activity in monoarthritic than in healthy rats. Combinations produced synergy of a supra-additive nature in the capsaicin test, but only additive antinociception in paw pressure testing. The efficacy of the drugs, alone or combined, is likely to depend on the differential sensitivity of tonic versus phasic pain and/or chemical versus mechanical pain to NMDA antagonists.

Decreased sensitivity to self-inflicted pain

There is anecdotal and incidental research evidence suggesting that self-inflicted injury is experienced as less painful than when the same injury is applied by another person. We tested the hypothesis that the sensitivity and the ability to tolerate pain differs depending on the person applying the painful stimulus. Self-selected healthy undergraduate students were obtained from the University of Stirling participant panel. None were suffering chronic pain or taking any form of analgesic drug. The participants applied a pressure algometer to themselves and to other participants. Depending on the type of trial, each was terminated when the participant experienced the algometer as either "painful" (for threshold reading) or "unbearable" (for tolerance reading). Both measures of pain, threshold and tolerance, were significantly higher when the algometer had been self-applied compared with when it was applied by another person. The mean difference for pain thresholds was 0.27MPa (95% confidence interval 0.10-0.44, P=0.002), and the mean difference for pain tolerance readings was 0.25MPa (95% confidence interval 0.03-0.48, P=0.028). An unexpected finding was that the mean tolerance score was less when females applied the algometer (P<0.01). When a painful stimulus was self-inflicted this resulted in significantly less pain and a greater ability to tolerate the pain compared with when the same stimulus was applied by another person. If the findings generalized to a clinical context, certain painful or discomforting procedures, such as mammography, removal of wound dressings and lancet withdrawal of blood, should be adapted for self-application by patients.

Eye-wiping test: a sensitive animal model for acute trigeminal pain studies

The possibility of introducing eye-wiping test as a model of acute pain was examined in rat, and it was compared with the well-known hot plate test. One drop of NaCl 5 M was placed into the animal eye, and the number of eye wipes with the ipsilateral forelimb was counted during 30 s. The withdrawal latency in hot plate test was also examined. Afterward, animals were treated with morphine (1, 2, 4, 6, 8 or 10 mg/kg), imipramine (25 mg/kg), sodium salicylate (250 mg/kg) or saline (i.p). After 30 min, the animals were tested again with eye-wiping and hot plate tests. Our results showed that morphine injection dose dependently decreased the number of eye wipes and increased the response latency to hot plate tests. There was a good correlation between the analgesic effects of morphine on responses to both tests, however, morphine produced more pain relief in eye-wiping test. Imipramine significantly decreased the number of eye wipes and increased the response latency to hot plate test, while sodium salicylate and saline injection did not. It may be concluded that the eye-wiping test can be used as a reliable method in trigeminal pain studies, which is sensitive to opioid and tricyclic antidepressant in rat.

The orofacial formalin test

The subcutaneous injection of formalin into the rat upper lip generates behavioral responses that last several minutes. The time course of the response is similar to what is observed following formalin injection into the paw, i.e. biphasic, with an early and short-lasting first phase followed, after a quiescent period by a second, prolonged (tonic) phase. The applied chemical stimulus (formalin) can be qualified as noxious since it produces tissue injury, activates Adelta and C nociceptors as well as trigeminal and spinal nociceptive neurons and is felt as painful in man. In addition, increasing the concentration of formalin causes a parallel aggravation of histological signs of tissue inflammation and injury. The measured behavioral response (face rubbing) is a relevant end-point: prolonged face rubbing is evoked by formalin but not saline injection and a positive relationship between the amplitude of the response and the formalin concentration is observed, at least up to 2.5%. At higher formalin concentrations, the use of other or additional end-points should be considered. Finally, the behavioral response in the orofacial formalin test is sensitive to various opioid and non-opioid analgesics. The orofacial formalin test can then be considered as a reliable way of producing and quantifying nociception in the trigeminal region of the rat.

The orofacial capsaicin test in rats: effects of different capsaicin concentrations and morphine

The aim of this study was to develop a rat model of capsaicin-induced pain in the orofacial region. We examined the effects of subcutaneous injection of different doses of capsaicin (0.25, 0.4, 0.8, 1.5, 2.5, 25, 50, 100, 500 microg) on the face-grooming response. Injection of capsaicin into the vibrissa pad produced an immediate grooming of the injected area with ipsilateral fore- or hindpaw. A positive relationship between the amplitude of the grooming response and the capsaicin dose was observed until 1.5 microg, but with the highest concentrations (ranging from 25 to 500 microg) the amplitude of the response decreased. Morphine administered either systemically (in the neck, 0.5-4.0 mg/kg) or locally (0.25-1.0 mg/kg) reduced in a dose-dependent fashion the face grooming provoked by subcutaneous capsaicin (1.5 microg). The systemic and local morphine effects could be reversed by systemic (0.1 mg/kg) and local (0.05 mg/kg) administration of naloxone, respectively. The local administration of morphine (ED(50): 0.65 mg/kg) was more potent than systemic injection (ED50: 2.54 mg/kg) in reducing the grooming behavior. In conclusion, the orofacial capsaicin test appears to be a valid and reliable method for studying trigeminal pain mechanisms and testing analgesic drugs. The results of the present study also support the clinical use of peripheral opioid administration for the treatment of orofacial painful conditions.

Orofacial deep and cutaneous tissue inflammation and trigeminal neuronal activation. Implications for persistent temporomandibular pain

A rat model has been developed to characterize the responses of brainstem trigeminal neurons to orofacial deep and cutaneous tissue inflammation and hyperalgesia. Complete Freund's adjuvant (CFA) was injected unilaterally into the rat temporomandibular joint (TMJ) or perioral (PO) skin to produce inflammation in deep or cutaneous tissues, respectively. The TMJ and PO inflammation resulted in orofacial behavioral hyperalgesia and allodynia that peaked within 4-24 h and persisted for at least 2 weeks. Compared to cutaneous CFA injection, the injection of CFA into the TMJ produced a significantly stronger inflammation associated with a selective upregulation of preprodynorphin mRNA in the trigeminal spinal complex, an enhanced medullary dorsal horn hyperexcitability, and a greater trigeminal Fos protein expression, a marker of neuronal activation. The Fos-LI induced by TMJ inflammation persisted longer, was more intense, particularly in the superficial laminae, and more widespread rostrocaudally. Thus, the inflammatory irritant produces a stronger effect in deep than in cutaneous orofacial tissue. As there is heavy innervation of the TMJ by unmyelinated nerve endings, a strong nociceptive primary afferent barrage is expected following inflammation. An increase in TMJ C-fiber input after inflammation and strong central neuronal activation may initiate central hyperexcitability and contribute to persistent pain associated with temporomandibular disorders. Since deep inputs may be more effective in inducing central neuronal excitation than cutaneous inputs, greater sensory disturbances may occur in pain conditions involving deep tissues than in those involving cutaneous tissues.

Medullary dorsal horn neuronal activity in rats with persistent temporomandibular joint and perioral inflammation

Studies at spinal levels indicate that peripheral tissue or nerve injury induces a state of hyperexcitability of spinal dorsal horn neurons that participates in the development of persistent pain and hyperalgesia. It has not been demonstrated that persistent injury in the orofacial region leads to a similar state of central hyperexcitability in the trigeminal system. The purpose of the present study was to conduct a parametric analysis of the response properties of nociceptive and nonnociceptive neurons in trigeminal nucleus caudalis (medullary dorsal horn, MDH) in a rat model of persistent orofacial inflammation. Neurons were recorded extracellularly and classified as low-threshold mechanoreceptive (LTM, n = 49), wide dynamic range (WDR, n = 82), and nociceptive-specific (NS, n = 11) neurons according to their response properties to mechanical stimuli applied to their cutaneous receptive fields (RFs). The inflammation was induced 24 h before the recordings by injecting complete Freund's adjuvant (CFA) into the temporomandibular joint (TMJ) capsule or the perioral (PO) skin. The mean areas of the high-threshold RFs of WDR neurons in TMJ (8.66 +/- 0.61 cm(2), n = 25) and PO (5.61 +/- 2.07 cm(2), n = 25) inflamed rats were significantly larger than those in naive rats (1.10 +/- 0. 16 cm(2), n = 32). The mean RF size in TMJ-inflamed rats also was significantly larger than that in PO-inflamed rats (P < 0.01). Furthermore the mean area of the RFs of NS neurons (3.74 +/- 1.44 cm(2), n = 5) was significantly larger in TMJ inflamed rats as compared with naive rats (0.4 +/- 0.09 cm(2), n = 3) (P < 0.05). The background activity in the TMJ- and PO-inflamed rats was generally greater in WDR and NS neurons, but less in LTM neurons, when compared with naive rats. The responses of WDR neurons to noxious mechanical stimuli were increased significantly in TMJ-inflamed rats (P < 0.05) as compared with naive rats. WDR neuronal responses to mechanical stimulation also were increased in PO-inflamed rats but to a lesser extent than in TMJ-inflamed rats. The injection of CFA into the TMJ or PO skin resulted in reduced responses of LTM neurons to mechanical stimuli. The responses of MDH nociceptive neurons to 48-55 degrees C heating were greater in inflamed rats as compared with naive rats. A subpopulation of WDR neurons recorded from TMJ (n = 4 of 10)- or PO (n = 3 of 13)-injected rats responded to cooling in addition to heating of the RFs but did not grade their responses with changes in stimulus intensity. These results indicate that persistent orofacial inflammation produced hyperexcitability of MDH nociceptive neurons. TMJ inflammation resulted in more robust changes in MDH nociceptive neurons as compared with PO inflammation, consistent with previous studies of increased inflammation, increased MDH Fos-protein expression, and increased MDH preprodynorphin mRNA expression in this deep tissue orofacial model of pain and hyperalgesia. The inflammation-induced MDH hyperexcitability may contribute to mechanisms of persistent pain associated with orofacial deep tissue painful conditions.

Paradoxical inhibition of nociceptive neurons in the dorsal horn of the rat spinal cord during a nociceptive hindlimb reflex

Nociceptive-specific and multireceptive neurons in the lumbar dorsal horn are excited by noxious stimuli applied to the hindpaw and inhibited by noxious stimuli applied to distant body regions. Given that at least a subset of these neurons are part of the circuit for nociceptive reflexes, inhibition of nociceptive-specific and multireceptive neurons should inhibit nociceptive reflexes. Unfortunately, previous attempts to test this hypothesis have been inconclusive because of methodological differences between electrophysiological and behavioral experiments. The present study overcame this problem by recording neural and reflex activity simultaneously. Rats were anesthetized with halothane and surgically prepared for single-unit recording from the lumbar dorsal horn. Hindpaw heat caused a burst of activity that reliably preceded hindpaw withdrawal in 10 nociceptive-specific and 17 multireceptive neurons. A distant noxious stimulus (tail in 50 degrees C water or ear pinch) inhibited the evoked activity of both nociceptive-specific and multireceptive neurons and simultaneously changed the topography of the hindpaw reflex from flexion to extension without altering reflex latency. The present data are consistent with previous reports of inhibition of nociceptive-specific and multireceptive neurons during application of a distant noxious stimulus. However, inhibition of nociceptive-specific and multireceptive neurons concomitant with a shift in the hindlimb reflex from flexion to extension suggests that these neurons are part of the circuit for flexor reflexes specifically. Presumably, lateral inhibition from the flexor to extensor circuit allows for the release of hindlimb extension when neurons in the flexion circuit are inhibited by a distant noxious stimulus. Such a system reduces the chance of injury by allowing for withdrawal reflexes to a single noxious stimulus and escape reactions, such as running and jumping, to multiple noxious stimuli.

Behavioral assessment of facial pain in rats: face grooming patterns after painful and non-painful sensory disturbances in the territory of the rat's infraorbital nerve

Noxious stimulation of the rat's face evokes intense face grooming with face wash strokes almost exclusively directed to the stimulated area (e.g. Clavelou et al., Neurosci. Lett., 14 (1989) 3263-3270). Similar asymmetric face grooming behavior has been observed after transection (Berridge and Fentress, J. Neurosci., 6 (1986) 325-330) and chronic constriction of the infraorbital nerve (Vos et al., J. Neurosci., 14 (1994) 2708-2723). In the present study, the relation between unilateral facial pain and asymmetric face grooming was experimentally studied in normal, intact rats: face grooming patterns evoked by non-painful sensory disturbances in the territory of the infraorbital nerve (i.c. unilateral vibrissae clipping, anesthetic infraorbital nerve blockade, application of mineral oil on vibrissae) were compared to those evoked by noxious facial stimulation (s.c. formalin injection in mystacial pad) and those observed in unstimulated control rats, using video-analysis. Only formalin-injected rats displayed significantly more face grooming activity directed to the affected infraorbital nerve territory than unstimulated control rats. Non-painful sensory disturbances (especially mineral oil application) induced an initial bout of directed face grooming; this response was transient and short-lasting. These observations suggest that directed face grooming can be used as a sign of unilateral facial pain in freely moving rodents; unilateral non-painful facial sensory disturbances do not lead to intense and persistent directed face grooming.

Direct comparison of heat-evoked activity of nociceptive neurons in the dorsal horn with the hindpaw withdrawal reflex in the rat

Although the sensory coding of nociceptive neurons in the dorsal horn has been studied extensively, surprisingly little is known about how these neurons contribute to nociceptive reflexes. The objective of the present study was to examine the characteristics of dorsal horn neurons capable of initiating hindpaw withdrawal. To this end, neural and reflex activity were measured simultaneously in response to noxious radiant heat applied to the hindpaw in lightly anesthetized rats. Subsets of both multireceptive (MR; 52/95) and nociceptive-specific (NS; 19/46) neurons showed a consistent burst of activity that preceded the reflex. However, when compared with NS neurons, MR neurons as a group were: more likely to be active before the reflex (55 vs. 41%); more active before the reflex (31 vs. 23 Hz); and active earlier (2.8 vs. 2.3 s before the reflex). Subsets of MR neurons were active before the reflex regardless of receptive field size or location in the dorsal horn. In contrast, NS neurons with small receptive fields or those located outside of superficial laminae were rarely active before the reflex and thus unlikely to be part of the reflex circuit. These results suggest that current classification schemes, in particular MR and NS categories, cannot be used as the sole criterion to predict involvement in nociceptive reflexes. However, simultaneous measurement of neural and reflex activity provides an opportunity to determine the characteristics of nociceptive neurons involved in withdrawal reflexes.

Quantitative analysis of orofacial thermoreceptive neurons in the superficial medullary dorsal horn of the rat

Surprisingly little is known concerning the central processing of innocuous thermal somatosensory information. The aim of the present study was to obtain quantitative data on the characteristics of neurons in the rat superficial medullary dorsal horn (sMDH) that responded to innocuous thermal stimulation of the rat's face and tongue. Single-unit extracellular recordings were obtained in chloralose-urethane anesthetized rats. A total of 153 thermoreceptive neurons was studied. Of these, 146 were excited by cooling and inhibited by warming and were classified as COLD cells. The remaining seven cells were excited by innocuous warming of the skin or tongue. Of 123 COLD cells tested, 33% were excited by touch and 22% by pinch stimuli delivered to the thermoreceptive field. Of the 50 COLD cells tested, 46% were excited also by noxious heating (> or = 50 degrees C for 5 s). Most (82/121) of the receptive fields were located on the upper lip, 25 on the tongue, and most of the remaining on the lower lip. Receptive fields were generally small (1-5 mm2). In some experiments, electrical stimulation in the thalamus was performed, and nine COLD cells could be activated antidromically. The responses of 38 COLD cells to incremental 5 degrees C cooling steps were examined quantitatively. Thermal stimuli were applied to facial or lingual receptive fields of sMDH neurons with a computer-controlled Peltier thermode starting from 33 degrees C, decreasing to 8 or 3 degrees C, and returning to 33 degrees C. Most COLD cells (26/38) had both static and dynamic responses; 7 had mainly dynamic and 5 mainly static responses to step decreases in temperature. Rat sMDH COLD cells could be classified into three groups depending on their stimulus-response functions. The first group (Type 1, n = 19) had a bell-shaped static stimulus response function. The second group (Type 2) had a high maintained or increasing static firing rate as the temperature decreased < 18 degrees C (n = 10). Type 3 COLD cells had mainly dynamic properties (n = 7). Many of the cells in all groups were excited by noxious mechanical stimulation. Type 2 cells differed from the other two groups in that most did not respond to noxious thermal stimuli (hot) and many responded to innocuous tactile stimuli. Neurons from each of the three groups of COLD cells could be activated antidromically from contralateral thalamus. These data suggest that there is little central processing of thermal information at the first central synapse for Type 1 neurons, however, the responses of the other two types may be due to central processing and convergence. The demonstration of rat sMDH COLD cells with distinctive stimulus-response functions to thermal shifts suggests separate functional roles of these neurons in the ascending thermal sensory pathway.

A pharmacological study of the modulation of neuronal and behavioural nociceptive responses in the rat trigeminal region

Electrical stimulation of the brain, particularly in the periventricular grey areas, caused long-lasting increases in behavioural escape thresholds to heating and mechanical stimuli applied to the facial region of the rat. The brain stimulation selectively suppressed responses to noxious stimuli. Responses to non-noxious stimuli, evoked by low threshold brush, were unaffected. The same animals that were studied in the behavioural tests were then anaesthetized with urethane and the inhibitory effect of the same brain stimulation was studied in single neurones recorded in the caudal trigeminal nucleus. A clear correlation (rs = 0.63) emerged between degree of behavioural antinociception and the amount of inhibition seen in nociceptive neurones. In addition the mean duration of the inhibition (6 min) was similar to the mean duration of the antinociceptive effect (7.3 min). Other classes of non-nociceptive neurones were unaffected by the stimulation. The neurones were also studied using iontophoretically applied monoamine candidates for the inhibitory neurotransmitter noradrenaline (NA) and 5-hydroxytryptamine (5-HT). The profile of the effects of NA most closely fitted that of the inhibitory neurotransmitter. This profile was expressed in terms of depression and excitation of different classes of neurones, and by the duration of effects. The depressants effects could be antagonized by iontophoretic idazoxan. In addition clonidine induced long-lasting depression of firing. 5-HT was more likely than NA to excite nociceptive neurones and to depress non-nociceptive neurons. Only NA consistently elevated thermal response thresholds in a similar manner to that produced by brain stimulation. These results provide some support for the hypothesis that selective descending inhibition of nociceptive responses in neurones of the rat caudal trigeminal nucleus is mediated by NA, possibly by an action at alpha2-adrenoceptors.

The orofacial formalin test in rats: effects of different formalin concentrations

In this study of the orofacial formalin test in rats, the effects of different formalin concentrations (0.2%, 0.5%, 1.5%, 2.5%, 5% and 10%) on the behavioural nociceptive response (face rubbing) was investigated. The histological responses of the skin were also evaluated. Increasing the concentration of formalin caused a parallel aggravation of histological signs of tissue inflammation and injury. All concentrations provoked an early phase of nociceptive response, but its intensity was not concentration-dependent. The 2nd phase of response to formalin only occurred for concentrations of 1.5% and higher. A positive relationship between the formalin concentration and the amplitude of the rubbing activity measured between 12 and 45 min after injection could be observed until 2.5% but with the highest concentrations (5 and 10%), the amplitude of the response decreased. Our findings indicate that the orofacial formalin test should be carried out using concentration between 0.5 and 2.5%. This is essential to assess increase as well as decrease in pain intensity. Moreover, this will have the effect of minimizing the suffering of the experimental animal.

Comparison of the effects of sumatriptan and the NK1 antagonist CP-99,994 on plasma extravasation in Dura mater and c-fos mRNA expression in trigeminal nucleus caudalis of rats

Dural plasma extravasation produced by electrical stimulation of the trigeminal ganglion was measured in rats and the concomitant expression of c-fos mRNA produced in the trigeminal nucleus caudalis (NtV) was measured using in situ hybridization techniques. The non-peptide NK1 receptor selective antagonist CP-99,994 (1-3000 micrograms kg-1) and the 5HT1D receptor agonist sumatriptan (1-1000 micrograms kg-1) reduced dural plasma extravasation dose-dependently with ID50S of 52 micrograms kg-1 and 30 micrograms kg-1 respectively. CP-99,994 (1000 micrograms kg-1). a compound known to have good brain penetration, decreased c-fos mRNA expression in the NtV by 37 +/- 7% without disruption of the blood brain barrier (BBB). Sumatriptan (1000 micrograms kg-1), known to be poorly brain penetrant, had no significant effect on c-fos mRNA expression in the NtV unless the BBB was disrupted by infusion of a hyperosmolar mannitol solution after which sumatriptan decreased c-fos mRNA expression by 65 +/- 11%. The results suggest that brain penetrant NK1 receptor antagonists may have anti-migraine effects peripherally through blockade of dural extravasation and centrally by inhibition of nociceptive pathways. Furthermore the data indicates that the anti-migraine action of sumatriptan must be predominantly peripherally mediated, be it via inhibition of plasma extravasation or direct vasoconstriction, since it had little effect on the activation of neurones in the NtV unless the BBB was disrupted.

Nociception and pain

In recent years, progress in the treatment of pain has been strongly influenced by new insights into the mechanisms underlying pain and nociception. The following article briefly reviews some recent reports which make a significant contribution to our knowledge of the structure and function of nocisensors, the neuropeptides involved in the nociceptive system, and the spinal and supraspinal central mechanisms of nociception.